Image Texture Size V's Mesh Size
One option that may work for you depending on what your objects are, their size, shape, etc. is to join the three objects together, unwrap the combined object so that the scale is exactly the same, and then split the object back into the original 3 parts. 

If you ever need to make any tweaks to your texture scale after splitting the objects simply use numeric input so that you can scale each UV map by the exact same amount.